In recent years, due to the rise of an interest in environmental pollution and the tightening of environmental regulations, a method using a filtration membrane, which is excellent in the completeness of separation, compactness, or the like, has come into the spotlight as a water treatment method.
A nonsolvent-phase separation method, which uses a nonsolvent-phase separation phenomenon making a porous by the phase separation of the polymer solution using a nonsolvent, is known as a method of producing a porous membrane used as a filtration membrane.
A wet or dry-wet spinning method (hereinafter, both spinning methods are collectively called as wet spinning) is known as the nonsolvent-phase separation method. According to this method, a porous membrane that has a large amount of filtrating flow rate, includes a good fractional layer, and is suitable for the treatment of a large amount of water is easily obtained.
When a porous membrane is produced by this wet spinning, a membrane-forming solution containing a hydrophobic polymer, a hydrophilic polymer, and a solvent is prepared first. Incidentally, the membrane-forming solution is discharged in an annular shape, and a porous membrane is formed by a solidification step of solidifying the membrane-forming solution in a solidification solution. Meanwhile, even though the membrane-forming solution may be introduced into the solidification solution via a free-running portion where the membrane-forming solution comes into contact with air (dry-wet spinning method), and may be directly introduced into the solidification solution (wet spinning method).
Since a hydrophilic polymer generally remains in porous portions of the porous membrane in the form of a solution, the porous membrane is dried after the hydrophilic polymer is eliminated by cleaning or the like.
For example, Patent Document 1 discloses a method of drying a porous membrane by a hot air circulation-type drying device as a method of drying the porous membrane. Specifically, a drying device in which hot air having a temperature, which is 60° C. or more and equal to or lower than the heat deformation temperature of a hydrophobic polymer, circulates employs a method of drying a porous membrane by allowing the porous membrane to continuously travel and sending hot air to the outer peripheral side of the porous membrane.